Certain medical implants and orthopedic implants require strength for weight bearing purposes and porosity to encourage bone/tissue in-growth. For example, many orthopedic implants include porous sections that provide a scaffold structure to encourage bone in-growth during healing and a weight bearing section intended to render the patient ambulatory more quickly. For example, metal foam structures are porous, three-dimensional structures that have been used in medical implants, particularly orthopedic implants, because they have the requisite strength for weight bearing purposes as well as the requisite porosity.
Metal foam structures and other porous structures can be fabricated by a variety of methods. For example, one such method is mixing a powdered metal with a pore-forming agent (PFA) and then pressing the mixture into the desired shape. The PFA is removed using heat in a “burn out” process. The remaining metal skeleton may then be sintered to form a porous metal foam structure.
Another similar conventional method includes applying a binder to polyurethane foam, applying metal powder to the binder, burning out the polyurethane foam and sintering the metal powder together to form a “green” part. Binder and metal powder are re-applied to the green part and the green part is re-sintered until the green part has the desired strut thickness and porosity. The green part is then machined to the final shape and re-sintered.
While metal foams formed by such conventional methods provide good porosity, they may not provide the desired strength to serve as weight bearing structures in many medical implants. Further, the processes used to form metal foams may lead to the formation of undesirable metal compounds in the metal foams by the reaction between the metal and the PFA. Conventional metal foam fabrication processes also consume substantial amounts of energy and may produce noxious fumes.
Rapid manufacturing technologies (RMT) such as direct metal fabrication (DMF) and solid free-form fabrication (SFF) have recently been used to produce metal foam used in medical implants or portions of medical implants. In general, RMT methods allow for structures to be built from 3-D CAD models. For example, DMF techniques produce three-dimensional structures one layer at a time from a powder which is solidified by irradiating a layer of the powder with an energy source such as a laser or an electron beam. The powder is fused, melted or sintered, by the application of the energy source, which is directed in raster-scan fashion to selected portions of the powder layer. After fusing a pattern in one power layer, an additional layer of powder is dispensed, and the process is repeated with fusion taking place between the layers, until the desired structure is complete.
Examples of metal powders reportedly used in such direct fabrication techniques include two-phase metal powders of the copper-tin, copper-solder and bronze-nickel systems. The metal structures formed by DMF may be relatively dense, for example, having densities of 70% to 80% of a corresponding molded metal structure, or conversely, may be relatively porous, with porosities approaching 80% or more.
While DMF can be used to provide dense structures strong enough to serve as weight bearing structures in medical implants, the porous structures conventionally used employ arrangements with uniform, non-random, and regular features that create weak areas where the struts of the three-dimensional porous structure intersect. That is, the conventional structure configurations lack directional strength and compensate for the weakness by making struts thicker, thereby decreasing the porosity, and conversely, a conventional structure with the desired porosity often lacks the desired strength because of the thinner struts. That is, the desired strength can be achieved in the prior art at the expense of porosity, or vice versa. There are no methods and/or products currently available that provide both the improved strength, improved porosity, and improved connectivity.
Further, trabecular bone structures are non-uniform and random in appearance on a micro-scale. It is also known that effective medical implants must be physiologically compatible with their surroundings in addition to providing the requisite strength, porosity and connectivity. Yet the conventional porous structures with uniform, non-random, and regular features that do not resemble trabecular bone structures. For example, U.S. Publication Nos. 2006/0147332 and 2010/0010638 show examples of these prior art configurations employed to form porous structures that exhibit the disadvantages discussed above, e.g., weak areas at the strut intersections, improved strength at the expense of porosity, and no trabecular features.
One way to enhance the effectiveness of an orthopedic implant may be to randomize the porous structure of an implant so it better simulates trabecular structures in which it is implanted. Therefore, in addition to strength, porosity and connectivity properties, it is believed that the performance of an implant with a porous structure could be improved if the porous structure could be randomized porous thereby providing a randomized scaffold structure as opposed to a uniform open cell structure. Methods known in the art to create randomized structures typically involve randomizing an existing uniform structure. These methods, however, are limited because they typically require manual manipulation of the struts, i.e., solid space, of one unit to match up with another unit to build a scaffold of desired dimensions. If the struts of the units do not match up, the integrity of the structure may be compromised if it has too many loose struts. Similarly, a randomized structure with poorly oriented struts may have poor distribution of residual stresses due to the manufacturing method resulting in warped or inaccurate parts. Accordingly, the structure of the initial units of the prior art, either identical or not, is usually simple to keep the stacking or building process manageable. Otherwise, building a scaffold from complex randomized initial units would be too time consuming and costly, particularly in computation expenses. Further, an additional drawback to randomizing an existing uniform structure is potentially making the structure weaker due to the unanticipated changes in the properties of the structure resulting from changes in the modulus and direction during the randomization process. Consequently, an original randomized structures, as opposed to a randomized existing structure, provides for improved strength along with improved porosity and enhanced complexity—e.g., trabecular features. As mentioned above, in the prior art, software applications typically produce porous structures that are predominantly uniform and regular. For efficiency, they repeat a small unit tile in the coordinate directions to fill a volume without gaps between the tiles. However, relatively few and simple shapes are employed within the unit tile due to the complexity of matching these tiles together.
Further, as a result of the deficiencies of metal foam implants and implants fabricated using conventional DMF methods, some medical implants require multiple structures, each designed for one or more different purposes. For example, because some medical implants require both a porous structure to promote bone and tissue in-growth and a weight bearing structure, a porous plug may be placed in a recess of a solid structure and the two structures may then be joined by sintering. Obviously, using a single structure would be preferable to using two distinct structures and sintering them together.
In light of the above, there is still a need for efficient methods to manufacture three dimensional porous structures, and the structures themselves, with randomized scaffold structures that provide for improved porosity without sacrificing the strength, improved strength including seamless junctions between units, and improved connectivity and having trabecular features.